Ch. 2

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Ch. 2 Anatomy of the X-ray Machine

• Radiology

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The X-ray Tube

• X-rays are produced in an x-ray tube.
• X-ray tube allows x-ray beam to be produced and
  controlled.
• Components of the x-ray tube
• Cathode (negatively charged)
• Anode (positively charged)

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Elements necessary for x-ray production

• 1. Source of electrons
• 2. Method of accelerating the electrons
• 3. An obstacle-free path for the passage of high
  speed electrons
• 4. A target in which the electrons can interact,
  releasing energy in the form of x-rays.
• 5. An envelope (tube) to provide a vacuum
  environment, eliminating the air molecule
  obstacles from the electron stream and preventing
  rapid oxidation of the elements.

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So what you need

• Electrons ??Accelerate ?? Clear path ?? Target
  ??Provide vacuum environment.

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X-ray tube components

• 1. Cathode (-)
• 2. Anode ()
• 3. Glass envelope encases these components and
  forms a vacuum.
• Vacuum- an area from which all air has been
  removed.

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X-ray tube continued.

• Target for collision between electrons and
  positive charged on anode is located on the
  anode.
• This collision produces heat (99) and
  x-rays/radiation (1).
• Window acts as doorway for the exit of x-rays.
• Entire x-ray tube is encased in a metal housing
  to prevent stray radiation and to protect glass
  envelope.

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Cathode

• Cathode provides source of electrons and directs
  these electrons toward anode.
• Components of the cathode
• Filament- coiled wire similar to a light bulb-
  emits electrons when heated.
• When heat is applied to atom, electrons become
  excited.
• Excitation- when electrons are moved to a higher
  energy level within the atom.
• This excitation forms an electron cloud that is
  then attracted towards the anode.

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Filament

• The filament is constructed of tungsten because
  of high melting point and high atomic number.
• The higher the atomic number, the more electrons
  that are available for excitation.
• Filament is housed in focusing cup and is heated
  by a low energy circuit.
• Milliamperage (mA)-amount of electrical energy
  being applied to the filament. Describes number
  of x-rays produced during exposure.

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Filament Continued

• Quantity of electrons released depends on heat of
  the filament.
• The hotter the more electrons, the greater the
  mA.
• Electrons are released but must then be
  accelerated for collision with target on anode.
• Acceleration is controlled by kilovoltage (kV)
  which is the amount of electrical energy being
  applied to the anode and cathode to accelerate
  the electrons from the cathode to the anode.
• Question What is the difference between mA and
  kV?

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Anode

• Basic construction is a beveled target placed on
  a cylindric base.
• Heat is an issue- copper acts as a conductor of
  heat and draws the heat away from the tungsten
  target.
• Temperatures greater than 1000C occur during
  x-ray production.
• How do we cool?
• Copper is at base of target.
• Surrounding glass tube with oil

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Types of Anodes

• Differences in anode type is associated with
  maximum level of heat dissipation possible.
• 2 main types of Anodes
• 1. Stationary Anode
• 2. Rotation Anode

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Rotating Anode
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Stationary Anode

• Is fixed in place. Found in dental and small
  portable radiography units. Have small capacity
  for x-ray production.
• Limitations
• Inability to withstand large amounts of heat.
• Repeated bombardment of target can cause damage
  to target.
• This damage causes pitting of the target
  surface.
• Have to have a large focal spot to accommodate
  higher temperatures.
• If target is damaged, may cause radiation to
  scatter in undesirable directions.
• This will cause lighter than expected x-rays.

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Rotating Anode

• Rotates through the center of the tube.
• Rotation provides a cooler surface for the
  electron stream.
• Helps to distribute heat over a larger area.

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Focal Spot

• The small area of the target with which electrons
  collide.
• The size of the focal spot has an important
  effect on the formation of the x-ray image.
• Larger the focal spot, the less clarity.
• Rotating anode can have small focal spot and
  still withstand a great amount of heat.

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Heel Effect

• More x-rays leave tube on cathode side of the tube

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Areas of Tube Failure

• Cathode Failure
• Filament failure- why does this cause concern?
• Cant heat cathode, no electrons produced.
• The pre-exposure button keeps this from ocurring.
  Filament is not heated to level needed until
  pre-exposure button is depressed.
• Anode Bearing Failure
• Bearings get damaged from heat
• Anode Target Failure
• Damaged from heat.
• Causes changes in density or blackness to vary
  among uses.
• To prevent this damage, high kVps and low mAs
  should be used.

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What is kVp?

• Kilovoltage peak (kVp)- the peak energy of the
  x-rays which determines the quality (penetrating
  power) of the x-ray beam.

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Areas of Tube Failure continued

• Glass Envelope Damage
• 1. metal deposits that form on inner lining of
  glass due to overheating.
• This process is called arcing
• 2. when air is present within glass housing
• Tube Housing Anomalies
• Rarely occurs.
• Can be due to shift in glass envelope within
  metal housing. Causes partially exposed
  radiograph.
• Oil leak can cause overheating.

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Technical Components of X-ray Machine

• Electrical
• High Voltage Circuit
• Low Voltage Circuit
• Timer Switch
• Rectification is the process of changing
  alternating current (AC) to direct current (DC).

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X-ray Tube Rating

• Dictates maximum combinations of kVp and mA
  without overloading the tube.
• Based on 4 factors
• Focal Spot size
• Target Angle
• Anode Speed
• Electrical Current

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Generator Types

• 3-Phase Generator
• Produces an almost constant electrical current by
  using 3 single phase currents.
• Most commonly used in modern x-ray tables.
• Produces low-energy x-rays so that radiation
  quality is increased.
• High Frequency Generators
• Causes many thousands of waves per second to flow
  to x-ray tube and then convert to radiographic
  energy.

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Other Components

• The Collimator
• Restricting device used to control size of
  primary x-ray beam
• The Tube Stand
• Supports x-ray tube during radiographic
  procedures.
• Shaky stands can cause motion artifacts

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The Control Panel

• Includes
• On/off switch
• Voltage compensator
• Kilovoltage selector (kV)
• Milliamperage selector (mA)
• Timer
• Exposure button
• Warning light

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